The Scientist: Michael T. Murray, N.D., co-author of the “Encyclopedia of Natural Medicine.”

The Answer: More than half—perhaps up to 70 percent—of Americans are deficient in vitamin D. If you’re not sure where you stand, get a blood test to assess your levels. Many doctors do these tests routinely now, so ask your doctor to do an annual check, or you can even order an at-home test.

READ MORE: Put the D in Your Diet

Ideally, your blood level of 25(OH)D (the specific form the test measures) should be between 30 and 80 ng/ml. There is no completely agreed upon threshold for what constitutes insufficiency or deficiency, but anything below 30 is enough to raise concern. (The Vitamin D Council says levels below 40 ng/ml qualify as deficient, testing labs often put the limit at 31, and the Endocrine Society says it’s 20.)

Vitamin D stimulates the absorption of calcium, so it’s vital for bone health and strength. It also plays a role in muscle function, the immune system, circulation, the respiratory system and brain development, and deficiency has been linked to an increased risk of cancer. You may not be getting enough D if you don’t spend much time in the sun, if you have darker skin, if you live in the northern U.S. or Canada (and therefore have fewer hours per day of overhead sunlight), or if you are elderly, pregnant, breastfeeding or obese.

A couple years ago, to combat the growing incidence of deficiency, the Food and Nutrition Board raised the recommended daily intake of vitamin D from 200 to 600 IU. It’s nearly impossible to get enough vitamin D from your diet, so to ensure adequate vitamin D levels, many experts now recommend supplementing with at least 2,000 to 5,000 units (IU) a day. That may seem like a lot, but to put it in perspective, the skin produces approximately 10,000 IU of vitamin D in response to 20 to 30 minutes of summer sun exposure. To get the same amount from milk, you’d have to drink about 100 glasses. Gulp.

READ MORE: The Vitamin D and Sun Debate